The titanium alloy Ti-6Al-4V has been used to make high strength fasteners, such as bolts and screws. The alloy is supplied in bar, rod, or wire form depending on the type and size of fastener to be made. Hitherto, the bar, rod, or wire has been supplied to the consumer in the annealed condition. The consumer forms the fasteners by such techniques as forging, heading, or extrusion, or a combination of those techniques. The fastener blanks are usually formed at an elevated temperature starting from about 1200° F. (649° C.), but usually not below about 800° F. (427° C.). The fastener blanks are then cleaned by immersion in a molten salt bath, which is followed with acid etching.
The fastener blanks are next heat treated to achieve a desired strength level. The known heat treatment is a two-stage treatment. In the first stage, the parts are solution treated, typically in an inert atmosphere, at about 1650 to 1775° F. (899 to 968° C.) for about 1 hour and then water quenched. In the second stage of the heat treatment, the parts are precipitation hardened by an aging treatment at about 800 to 1050° F. (427 to 566° C.) for 2 to 8 hours and then cooled in an inert gas or in a vacuum. Hitherto, the aging step has been performed directly after the solution-treating step.
The solution treatment is the most problematic step in the heat treatment cycle because during solution treatment contamination of the fasteners must be prevented. Titanium and its alloys are very reactive, especially at elevated temperatures such as those typically used for solution treatment. Any foreign material which comes into contact with the titanium or titanium alloy during solution heat treatment will result in contamination of the material. Common sources of foreign material in the solution treating process are contaminants in the furnace atmosphere or residual processing materials such as lubricants on the surfaces of the titanium blanks. In order to avoid the problems associated with the presence of such contaminants, special furnace atmospheres must be maintained and the fasteners must be thoroughly cleaned before they are placed in the heat treating furnace. Cleaning of the fasteners presents another problem because it involves the use of aggressive chemicals which pose environmental hazards and disposal concerns. Additionally, the cleaning operation can change the chemistry of the fasteners, such as by adding hydrogen, and can alter the dimensions of the fastener by dissolving metal from the blank. These problems make the cleaning step unreliable, time consuming, and costly.
The known solution treating operations are also troublesome because the process often results in the final parts being unacceptable. In some cases the parts become contaminated with impurities because of less than desirable furnace atmospheres or residual lubricants on the parts. In other cases the parts have poor mechanical properties because of inadequate quenching. Moreover, parts can become distorted or they may stick together during solution treatment, or they may become bent or develop flat spots. It is also a fact that solution heat treating equipment is expensive and costly to operate for parts manufacturers. Elimination of the need to solution treat the headed fastener blanks, along with the associated relaxation of cleaning requirements, would permit significant improvements in the efficiency of the manufacturing process and greater uniformity of product quality.
Fasteners are also manufactured by direct machining of solution treated titanium alloy bar. The machined fasteners are then age hardened. However, that process does not involve a forging operation. Therefore, it is not susceptible to the problems discussed above.
Beta titanium alloys, such as Ti 3-8-6-4-4 and others, are supplied in the solution treated condition for forming into fasteners. The fasteners are then aged to achieve the desired properties. However, there are significant metallurgical differences between the beta alloys and the other known titanium alloys such as alpha, near-alpha, an alpha-beta alloys.